In the various types of data recording systems based on use of a magnetic medium, a magnetic transducer head is used to create the magnetic patterns in the medium and to read back the data after recording it. The typical magnetic head has an 0-shaped core element which provides a magnetic flux path including a flux gap across which flux fringing occurs. During reading and writing, the flux gap is placed in close proximity with the medium, and the medium is moved with respect to the gap. The fringing flux during writing enters the medium, causing the magnetic patterns to be created therein. The flux flow for the writing process is created by a winding through which the flux path passes.
During reading, the medium is again moved relative to the gap, and the magnetic pattern in the medium causes flow of flux in the flux path, which flux flow recreates the magnetic pattern in the medium. By detecting, i.e. converting it to an electrical signal, this flux flow can be converted into the original data. During reading, voltage across this same winding can be used to detect the flux flow, or a separate flux-detecting element such as a magneto-resistive element can be used.
There are two types of magnetic recording heads now in general use. The older type has a ferrite or other core providing the flux path and a separate wire winding for writing and reading. The newer is formed by photolithographic processes and is typically referred to as a thin film recording head. The invention forming the subject matter here is concerned with the ferrite core type of recording head.
The complete O core of a ferrite core head typically comprises a C core element, so called because it is in the shape of a C, and an I core element having an elongated rectangle shape. The I core and C core elements are attached to each other in some manner with the I core element connecting the arms of the C core element to thereby close the flux path. One of the connecting points between these two elements forms the flux gap. Typically, the flux gap is formed of a hard non-magnetic material such as glass or alumina. The two facing surfaces of the I core and the C core which define the flux gap are called the gap faces.
In most applications and certainly for disk drives for digital storage of data, it is very desirable to make the width of the flux gap (dimension of the gap faces which is adjacent the medium and parallel to the medium surface) very small so that the magnetic patterns created by writing are confined to a very narrow track. By reducing the gap width to 0.001 in. (0.025 mm.) or less for example, it is possible to space tracks of data on a disk so that 1000 or more are packed in a radius of one inch. It is preferred to closely pack these tracks so as to increase the amount of data which a single disk drive unit can store, thereby increasing the compactness of the drive itself for a given data capacity. It also turns out that in general by increasing the capacity of a disk drive by increasing the density of data storage on the individual disks as technical advances allow, one can on a cost per bit basis store data more cheaply than is possible by simply increasing the number of disk drives in an installation.
To make the individual cores used for the heads, a C bar element whose cross section is uniformly identical to that of an individual C core element, is bonded to an I bar element whose cross section is the same as that of the individual I core elements. Usually the I and C bars are from 50 to 100 times as long as the finished core width, with the gap width substantially smaller than the core width. A thin layer of gap material is placed, now usually by a sputtering process, at the interface corresponding to the gap of the finished cores. The I bar is then clamped to the C bar to close its ends, in combination with it assuming the shape of an O bar. The I bar/C bar assembly is then heated to a temperature sufficient to fuse the gap material to the adjacent I and C bar gap faces, fixing the gap length and bonding the bars to each other in the O bar shape. The assembled bar resulting is then sawed transversely into individual cores which can be wound and mounted in a suitable support. For use in rigid disk drives, these cores are mounted in slots in hard ceramic sliders which are designed to aerodynamically fly in close proximity to the disk surfaces.
The trend is to smaller gap lengths. (Length of the flux gap refers to the dimension in the direction of the flux flow in the core adjacent the gap, and is essentially the thickness of the gap material.) By making the gap length very small, it is possible to write and read individual flux pattern changes which are packed very closely together, increasing the amount of data which can be stored in a given length of track. Increasing this linear bit density again increases the overall capacity of a given amount of medium area to store data. Here too, it usually is economic to increase the linear bit density to what the state of the art allows, since this results in reduced costs for storage of a given amount of data.
It is also necessary to accurately control the gap length, since gap length interrelates with the other dimensions and parameters of the medium, so that deviations from the design gap length can adversely affect performance. For example, incorrect gap length can substantially affect the flux pattern created during writing or the electrical signal generated during reading, causing data to be read incorrectly.
Control of gap length has always been a difficult problem because of the small dimension involved. It is now customary to make the gap on the order of 20 .mu. inches (about 0.5 .mu.) long, and reliably reproducing such a small dimension in a manufacturing process is very difficult. The tolerances are now typically held to .+-.5 .mu. in. (.+-.0.1 .mu.). In particular, where the I and C bars must be bonded with the gap lengths controlled to within say 15-25 .mu. in. over perhaps 50 to 100 core widths or 1 in., it is virtually certain that using current technology, much of the length along the assembled bar will have improper gap length. This occurs for two reasons. In the first place, the two bars may have gap face areas which have not been machined to interface within just a few .mu. inches of perfect contact along the entire length of the gap faces. This arises from unavoidable flaws in the lapping process which is intended to produce perfectly flat gap face areas on the I and C bars. Secondly, clamping with conventional point contact clamps and then heating in the bonding step allows the individual bars to distort such that even initially perfect and uniform contact becomes incorrect. In either case, many of the individual cores which are later cut from the bars will have gaps with improper lengths, or the bonding between the gap material and the ferrite will be imperfect. These cores must be scrapped, since there is no way to correct these flaws. Thus it can be seen that current processes result in large numbers of defective cores which must be scrapped. Although there is not a great amount of the total cost of a finished head expended at this stage of the manufacturing process, it is much preferable to have high yield at this, and each, step of the process, since the likelihood of defective parts slipping through the inspection process after the step is done is reduced substantially whenever the process has only a small fraction of defective parts to begin with. In fact with sufficiently high yields, individual inspection of the parts after every step is unnecessary. And regardless of the cost of individual parts at even an early stage of the process, the defects still are an expense.
There are a number of different ways in which the gap may be formed in a manufacturing process. In one process, the I and C bars are positioned so that the spacing between them is equal to the desired gap length. Molten glass is then applied to the gap so that capillary action draws the glass into the gap simultaneously forming the gap and bonding the I and C bars. Another way involves placing a glass foil between the I and C bars to define the gap, and then heating the assembly and applying mechanical pressure to it to melt the glass and bond the bars mechanically. Yet another way is to place a glaze paste on the gap faces of the I and C bars and then fuse the paste to the gap faces with heat to form the gap. Another process uses a nonmagnetic and abrasion-resistant layer to define the gap and then places a bonding material in the form of a thin cane or wire at the inner line of the gap within the I and C bar assembly. Heat is then used to fuse or bond the bonding material to the I and C bars. U.S. Pat. Nos. 3,452,430; 3,550,264; 3,395,450; 3,375,575; 3,024,318; 3,117,367; 4,536,270; 3,233,308; 3,246,383; 3,767,497 and 3,333,332 disclose these processes with a number of variations. In all of these processes, the inability to accurately control the viscosity of the glass gap material by control of the temperature, and the variation of the pressure along the I and C bar interface during bonding results in gap lengths which are difficult to control sufficiently to achieve high yields.
The later approach, as was briefly mentioned earlier, forms the gap from alumina rather than a glass layer. Alumina being very hard, is fully the equal of glass as far as resistance to erosion and undercutting during machining or abrasive contact in use. But alumina has the additional advantage of chemical and mechanical stability during the relatively high temperature bonding process. Glass tends to flow under pressure and heat, and with the extremely short gap lengths involved, results in extremely variable gap lengths whereas alumina is much more stable. The alumina gap material is sputtered onto one or both gap faces of the I and C bars, as is explained in U.S. Pat. No. 4,536,270. Sputtering is inherently capable of depositing very accurate, uniform and repeatable layer thicknesses. When formed by sputtering, alumina layers, or layers of any material for that matter, are strongly adherent and completely conform to the substrate surface.
The present conventional processes then use a thin layer of bonding glass sputtered on at least one of the surfaces to be joined, and the I and C bars properly juxtaposed and subjected to heat and pressure to complete the bond. Unfortunately, to bond alumina requires relatively high temperature, leading to dissolving of the ferrite by the bonding glass, and an indeterminate gap length. However, avoiding this problem by use of very thin layers of glass as the bonding agent between the alumina and the other face makes it difficult to assure that solid bonding occurs along the entire length of the bars because of the hardness of the alumina. This is because of unavoidable spacing variations or voids between the two faces at which the bonding occurs due to deviations from perfect flatness resulting from mechanical and thermal stresses and imperfections in the lapping process preventing precise flatness of the surfaces to begin with. However, the mechanical rigidity of the alumina prevents its flowing to fill the voids between the bonding faces as does glass, in effect substituting one disadvantage for another.
As a general observation, it is desirable to effect the conventional core assembly process with as low temperatures as possible to avoid the possibility of damaging the magnetic properties of the ferrite or causing mechanical damage through thermal stress. This also reduces the magnitude of reaction between the ferrite and any adjacent dissimilar material. It is known that core bonding at relatively low temperature can be done by increasing the pressure between the faces of the parts at the bonding interface of the parts to be joined. This is called diffusion bonding or thermal compression bonding. Such processes are described in U.S. Pat. No. 3,672,045 and in IBM Technical Disclosure Bulletins Vol. 20, No. 10, March 1978, p. 4088, by Daniels et al., "Diffusion Bonding of Dissimilar Ceramics"; Vol. 21, No. 6, November 1978, pp. 2212-2213 by Kehr et al., "Ferrite-Ferrite Diffusion-Bonded Recording Head"; and Vol. 24, No. 3, August 1981, p. 1496 by Chow et al., "Diffusion Bonding Fixture".
However, it is difficult to cause the individual surfaces of a pair of ferrite bars to intimately contact each other along the entire length of their bonding interface, and if there is not this intimate contact the bond is faulty. A solution to this problem is provided in IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 1503-1505 by Rigby, "Diffusion Bonding of NiZn Ferrite and Nonmagnetic Materials", where a what will be hereafter referred to as a "granule vise" is described. By burying a pair of ferrite bars with their surfaces to be bonded juxtaposed, in a volume of hard, heat resistant granules contained in a cylinder bore and then powerfully compressing the granules with a piston driven into the bore, the bars are strongly forced against each other. By applying heat to the bars after thus compressing them against each other, high quality diffusion bonding can be effected along the entire length of the bonding surface of the bars.